1. Field of the Invention
The present invention relates to an optical disk of the type from which the recorded information is reproduced by focusing a laser beam thereon with an objective lens and detecting the reflected light beams with photodetectors. Also the invention relates to methods of information recording, reproducing, and optical disk management for preventing its illegal use.
2. Description of the Related Art
The use of multiple data layers is an effective way to increase the capacity of an optical disk. Different variants of such disks were proposed in many patents (for example U.S. Pat. Nos. 4,090,031; 4,450,553; 5,126,996; and 6,241,843). The semitransparent layers that were used there are thin metal films placed inside the transparent polymeric material. Each layer contains the preformed tracking grooves with information pits recorded inside each groove. The reading beam is scattered by pits and is reflected by the smooth parts of the layer. The relief depth of the data layer is less than one micrometer, while the usual disk thickness is 1.2 mm. Therefore, the employment of the third dimension has great potential for the enhancement of the disk capacity.
However, the technological development in this direction is limited due to the interlayer cross-talks during the optical reading. To reduce the interlayer cross-talks the distance between layers should be increased. On the other hand, there are difficulties with the spherical aberration at the large interlayer separation: the marginal light rays are focused at a higher point than central rays due to the influence of the substrate. The aberration can be compensated only for a certain distance from the optical disk surface to the plane of focusing (U.S. Pat. Nos. 5,251,198 and 5,625,609). For this reason the thickness of the transparent slab of the main disk body, through which the beam passes, must not vary by more than 100 micrometers. That limits the number of data surfaces, which could be practically realized. Thus, the above obstacles currently prevent the design of new 3D optical disks with a super-high capacity.
V-shaped grooves were proposed for the single data layer optical disks (for example U.S. Pat. No. 4,534,021) where the neighbor grooves containing the information pits were inclined in opposite directions. This allowed to decrease the cross-talks between the neighbor grooves and gave the possibility of increasing the disk capacity.